Polyethylene air filter



Patented Oct. 7, 1952 UNITED ,v STATES i PATENT UFFICI-.lf1

POLYETHYLENE AIR FILTER WilliamH. Nicol, Cuyahoga Falls, Ohio,assig'nor l to `Wingioot Corporation, Akron, Ohio, a corporation ofUD'elaware Application March29, 1950, Serial No. 152,670 s claims'. (c1. 18s-.45)`

This inventionrelatesto lters forthe removal` l of particleswofioreign matter from gases.-

A majorgproblem in conditioning,` air for varit oususesresides in the economical yand eiiicient removal of foreign-matter presentin the atmos-` phere:l This `foreign matter may be that` which is normally present rin the ,atmosphere and which is desirableto remove asa step in conditioning the .airgfonuse in -a.: home, a. hotel or similar liv; ing accommodation where lthe presence of such.`

foreignmatter is injurious to human healthier to walls, ldrapes, fiioor coverings: furniture, and

the like; or in a shop, `factory or laboratory where it is desirable to work under dust-free conditions It mayV also vbe :desirablejto remove fromwthe` atmosphere wforeign matter not Vnormally pres-v entwtherein,Y Aas for example in manufacturing t operations involving the presence of paint fumes and; `similarly nely divided materials.

A wen-known-method `of conditioning air-by removing particles of foreignzmatter therefrom is in theuse of animpingementtypeof yfilter Az comprising a supporting medium of steelcwool; mineraliwool; excelsior or glass bers coated with an'adhesive, such as petroleumoil, tricresyl'phosphate, or ethylene glycol. The gas is'conditoned i by lforcing Vit through Aa' matof the vsupporting material and the particles of foreign matter are;` Y

removedvfrom the gas should-:they impinge `upon the adhesive;` 'Ihe` effectiveness of an impngementntypeyof, iiltendepends uponthe opportunity i a dustparticle `has'to strike a surface vcontain-.f

It is not. practical "to expose ing the adhesive. too 'much surface per unit` volume to the `gas to befconditioned, because` the energy 'consumed in forcinggthef.gasipastvthese surfaces .would `be t thevsmallfparticle size 'rangegets' throughthe 'i filter, and it is. these small particles that cause walls,` drapes; floor coverings and furniture to becomedirtyifand require cleaning `in one way or another.w-;If allor a great percentage of this air,the-walls, drapes, floor coverings and furni-` f turein aroom so conditioned would not need to be'cleane'd of vthis short of dirt, which is the primarycause for cleaning.

In addition to (being ineffective in removing the 1fineparticles of foreignmatter from the' air,

the impingementtype lof lt'er also has atendency to contaminatethe air :passed therethrough because of the removal .of a; portion vof the-adhesive carried `by the `supporting a material `through@l I evaporation or physical disengagement therefrom. It has also-'been reported that in iilters `of' the glass fiber variety; small strands :of -the glassl `f fiber have a tendency to vbecomeifreefand thusgf.:

pass Vinto the air `stream and on into the'` roo being conditioned` f Another important:disadvantageqpossessed.by l thefimpingement type Aof iilteris thegfactf thatf 4 they are not reusable in an emcientiand economical manner. If anattemptis madewto clean` this type of filter with a detergent; the adhesiveis removed `from the;berglassa1ong with-the impinged dirt particles. -In this-conditionfthel filter is practically` useless as aiilter. .Beforethel g iilterV` can be i `restoredV `to l its' i original ltering 1 ability, it is necessary to recoatwthe fiber-glass.` withanadhesive. Thisis an expensive and` l time-consuming operation Aandf .one` that is` not readily-done by the.user. t

A more efficient. means that .may` be used in removing. foreign matter from gases is the .electrical precipitator. YHoweventheseprecipitators l are prohibitivelyexpensive for manyuses, par ticularly for conditioning airinhomes. They are expensive .because of their `initial cost-and l because of their upkeep. l.

There has. now been discoveredv a' means for, removing nne particles of foreign matter from .A gases whichis` substantially asefiicientas the t, electrical precipitator but approaches the cost` The iilter. medium of thisinventionlcomprises polyethylene` arranged to expose a largenumberofsurfaces of the impingement ,type of filter.

at I comprises aframez@ made of anylsuitable material vsuch as cardboard. steel; aluminum` and the 'like shapedfto form a channel section having a side 3 joiningthe sides 4 and; 5; each` disposed at right angles to the" side 3. Theiiame may.A be fabricated 4from a strip of aluminum-.by cut-w` 1 ting i out the ynecessary materialtin order 'to forma the corners 6, then having a portion of the material turned up to form the sides 4 and 5, and then pressing out a portion of the side 3 to form a catch 'I to receive the closing side 8 of the frame through the latch member 9. The side 8 is a channel section similar to the frame 2 which is heldin place as shown. Disposed within thesides 4 and 5 are two'screens l0 and Il having any suitable mesh opening, preferably about 1A", between which screens is placed the polyethylene I2 shredded into ribbons of various widths and lengths and arranged to present a porous mat through which the contaminated gas is passed in order to bring the gas into contact with surfaces of the polyethylene.

It is desirable in using polyethylene as a filter medium to arrange the polyethylene in such a way as to expose a great many surfaces thereof to the gas passed therethrough so that the gas will come in contact repeatedly with surfaces of polyethylene. The polyethylene may be arranged in thisv manner by shredding or cutting polyethylene film into fiat ribbons of any particularlength, and of awidth ranging from 1/64 of an inch and less to 1/8 ofl an inch, and ranging in thickness from 0.002 of an inch toV 0.008 of an inch. Generally the ratio of the width to the thickness of a ribbon of polyethylene will vary from5 to `1, to 100 to l. Fig. 2 shows a polyethylene element having a ragged edge I3 formed bycutting a sheet of lpolyethylene into ribbons. The nature of the polyethylene is such that there is a tendency for the polyethylene to tear and i'orm a ragged edge as it is being cut into ribbons.

The polyethylene may be used in other forms. For example, instead of cutting a flat sheet of polyethylene into ribbons, filaments or fibers of convenient cross section may be used. Fig.' 3 shows a mat of polyethylene filaments I5.- Polyethylene in tubular form may also be used. Another form in which the polyethylene may be used is that in which a sheet of polyethylene is lacerated by forming a series'of parallel slits of any convenient length, for example, 6". Intermittent areas of laceration are made throughout the length of polyethylene film, which lacerated film is then folded into a convenient frame until the desired thickness is attained. It is desirable in making these lacerations to cut the film in such a way as to form a fine ragged edge. These ragged vedges will then function tomechanically hold particles attempting to pass through the slits or lacerations in the film. 4Particles of polyethylene, either spherical or of an irregular shape, may also be used, since the polyethylene in this form permits repeated contact of the lgas With polyethylene surfaces when the gas is passed through a mass of these particles.

When an air filter using polyethylene is .made as shown in the drawing and used in combination With a conventional impingement type of filter in an air conditioning system, an unexpected ltering phenomenon is observed. Illustrating this are the following tests'. A polyethylene filter was made inv accordance with the description of the filter shown in Fig. 1 of the drawing, in which a frame 20" x 20 was made and provided on one side with a screenhaving 1/4'.' square openings. Ribbons of polyethylene of lvarious, length, an average width of als and .006 thick, .were sprinkled onto the screen evenly to a depth of l. This mat of ribbons was held inplace with a second screen having 1A square openings andsecured to the ,frame opposite to the first screen all as shown in the drawing.

In the first test a commercial filter of fiber glass coated with tricresyl phosphate and being 20" x 20" X 1" in size was placed on top of the polyethylene filter and the two filters were inserted in an yair stream so thatthe air stream I through the commercial iilter.

A second test was run using the same type of' filters but this time the polyethylene filter was positioned on top of the commercial filter and the two filters then inserted into the air stream in such a manner that the air first passed through the polyethylene filter and then through the commercial filter. After 786 hours of service, in the same machine under conditions identical to the conditions used in the rst test, the filters were removed and inspected. The polyethylene filterwas dirty and discolored, but the surprising thing was that the commercialA filter was substantially the same color as it was when it was first placed in use and as freev from visual dirt as it was when it Was first inserted into the air stream.

In attempting to explain this unexpected result, it was observed that the polyethylene possessed an ability to develop an electrostatic charge on its surface and that this electrostatic charge was produced when a gas was caused to pass over a surface of polyethylene. Thus, the remarkable efficiency of the filter of this invention appears to be due to the fact that the'polyethylene used in these filters is capable of developing an electrostatic charge sufficient to attract fine particles of foreign matter contained in a gas, and that the polyethylene is shaped and arranged in such a manner as to permit repeated' contact of the gas with polyethylene. By actual measurement it is found that polyethylene will develop an electrostatic charge of about 250 volts per 1.5 square inches of surface when air is passed over a polyethylene surface ata velocity of up to 1200 feet per minute.

It was also observed that the filters of'this invention become electrostatically charged upon the first passage of air thereover and retain the foreign matter collected until the foreign `matter is removed in some convenient manner, as by cleaning with a detergent and water. It was also observed that an electrostatic charge is developed by the action of cutting a film of polyethylene into ribbons. The filters of this invention are not sensitive to moisture and function equally well with dry gases as Vthey do with gases having a high moisture vapor content.

For the purposes of this invention, all solid ethylene polymers may be used. Various methods for preparing these polymers are known. For example, these solid polymers of vethylene can be prepared by subjecting ethylene to a pressure in excess of 500 atmospheres' under controlled, elevated temperature conditions This procedure results in polymers of ethylene, the molecular Weight of which varies, depending particularly upon the pressure employed. By using pressures Y .zaeraoco of more `than 1000 atmospheres, :solid polymers lrare'formecL and under theseconditionsthe poly- -merzation action takes place, requiring several nhours'for completion. These .polymers canalso -be .made by including in `the ethylene ha small l quantity of oxygen `which can ibe *as little .as

n0.01%,1but preferably 0.02% `to\0;l0% atfzl500yatlmospheres. npreferably atleast 1000 atmospheres, and'below '3000 atmospheres. 4100 GC. to 4400" C., .and more desirably from The `pressures are Vat least500, :and

The `temperatures .are :from

150 C..to 250 C. Thus, asa speciclinstanceof based on theethylene used, is obtained.

The `ethylene polymers obtained .as .outlined above melt at temperatures in excess `oi 100 C.

'.andgenerallyinthe range of about .110 C. to .:about.200.C.,. have .amolecular Weight in excess of 6000, arefessentially saturated .products `corre- -sponding in .composition substantially to l (CH2) e `@and Whensulojected` to yX-ray .diffraction 'analysis show a.crystallineistructure- These polymers are .insoluble .inxyleneat.ordinaryxtemperatures but soluble 'in Xylenef at tsboilingpoint, `and are unaffected by prolonged contactwith airat ordinary Avtemperatures, `thus making `them .admirably adapted'for usein the air lters of thisinvention.

'The polymers .are'iurther characterized :by the factthatthey canbe formed into iilms'and rilaments and yield oriented products on application Lof-.stress referredto as cold drawing.

Thus it :ha-s been .rfound :that 'both'fthe oriented Y.andunorientedzproducts can be .usedin thepractice of thisinvention. Oi these "polymers,` polyiethylenes. having a. molecular .Weight in excess of vv10,000 are preferred, `and polyethylenes `having a molecular weight in the range of from 10,000

/to 20,000 all function .exceptionally `well when yused in accordance with the `practice vof .thisinvention.

. Any ethylene .polymer capable of .developing :an :electrostatic charge .may :be .used in thisl inventiOn.

polymerizing `ethylene in the :presence of .other 4monomers Vcopolymerizable with ethylene. These ethylene polymers. can also be used. inithis. inven- .tion where they .have the ability'to `develop .an

.electrostatiocharge. "Specicmethods of making -1 ethylene polymersare described in U. ,S.Patents :.Nos. 2,153,553; 2,212,786; `.and 2,261,757.

The following examples show'a oomparisonlof the eliciency ofza :filter medium of the present invention and a commercially available ilter medium of the impingement type. In the examples all parts are by Weight unless` otherwise r identied.

A filtration cell was madelby filling a container i .inches in diameter and2 inches high with a vquantity; of glass fibers coated with tricresylphosphateas the adhesive. made by. lling a container 4 inches in'diameter Another lter "cell `was and 2 inches high with shredded polyethylene having a molecular Weight of about 18,000. The polyethylene had anaverage width of g1g of an inch. The :lteringxmaterial was looselynpacked j .Ethylene `polymers .are `also made by in the .container and enough used to `produce a loose mat-.of polyethylene through which the. air could beeasily passed. The gas entrance side andthe gas exit side of the containers wereeach ifi-tted with a metal screen having 1/2 inch square 'openings tofkeep the lter material in place. The lter `containing coated sglass bers was placed in an air` stream having. a velocity of 300 feet per minute. A quantity of 3.1212 parts of Super- Spectra carbon black having an averageparticle *sizeof '0.013 microns `was then dispersed inthe `airstreamlby means of a duster over aperod of 30 minutes. A quantity of 0.2598part of the carbon black was collected. Thus, the coated fiber filter medium had an elrlciency of 8.3%.

The lter containing shredded polyethylene -wasinserted in the airstream having `a velocity of `300 lfeet per second. `A. quantity oi' `3.2034 par-ts of the'same carbon black` was `dispersed in 4thealrstream over a `.period of 30 minutes `and 1.9433 parts .was-collected, indicating `aneil-ciency lO'f 64.30%.

Example i2 Acommercial `fllte1"(20" x 20 X 1) containing glass "bers coated with tricresyl phosphate was used in thistest and modified by removing ,1/2 of thefilter medium or an area l0" x 10 xl",

and `replacing it With shredded polyethylene'having anV averageribbonwidth of of anlnch and molecular weight of about 18,000. .The ribbons ofpolyethylene were `loosely packed into the frame ofthe fllterand enough used to producea loose mat of polyethylene through vWhich the air could be easilyv passed. This lter Was placed inservice in a forced draft `air circulating system operating .on air inthe vicinity of The Goodyear VTire lrRubber` Company, Akron,'O=hio. Afterbe- `ingin service continuously for 1188 hours, the filter Vwas removed and examined,

` 'It was observed Lthat the glass ber section of the lter Was `almost completely choked up with particles foff'oreignlmatter onthe air entrance side and practically free'o'f "foreign matter on the air exit "side .otfthe'lten It was further lobs'ewedthat 'substantially all of the `foreign matter onftheair 'entranceside of 'the glass `ber section of the *filterwas conned to the surface with verylittle vforeign matter -to befound le of an inch into the depth of the iilter.

The polyethylene section of the filter was not choked with foreign matter on the air entrance side of the lter and it Was observed that the foreign matter had been substantially equallydis- '.tributedthroughout the thickness of the massief polyethylene. This type of distribution.' of 'the dust permits greater quantities of dust to be colilected andlessens the tendency-for the filter to folog. "Thisffunction `is not observed `for theirn- Lpingementtype of .lten

It was further observed that the pressure drop on the polyethylene section of the lter was substantially the same at the end of the test as it was fat lthe beginning of thetest, whereas the pressure =drop forlthe glass'ber section of the filtergradl :ually-increaseds constantly throughout l its time 'of use.

AAEach section of. `the 'filter Was removed and washed separately in equal volumes of Waterfcontaining thesame detergent. The Washings Were :strained to remove pieces of Yglass and polyeth- `ylene'ibersand then placed in bottles to observe .the nature ofthe suspended foreign'in'atter. The .foreignmatterlfrom `the polyethylene iilterhad a blackerappearance than thatobtained frornithe i glass ber section of the filter. sedimentation tests on these suspensions showed that the polyethylene had removed a large amount of colloidal carbon from the air which was almost entirely absent in the washings from the glass fiber section of the filter. After settling for one month, the washings from the glass fiber settled out almost completely, leaving a clear supernatant liquid above the sediment, while the washings from the polyethylene section of the filter were still black.

The following example shows-the effectiveness of the filter of this invention in removing tobacco smoke from the air.

Example 3 A smoke generator was used consisting of a glass tube of a length and diameter to receive 10 normal-sized cigarettes and placed end to end so that the combustion zone would pass from one cigarette to the next in the direction of the air stream. The end cigarette on the air entrance side of the tube was then ignited and air was drawn through the tube at a constant rate. The smoke-filled air leaving the air exit side of the tube was passed through a piece of rubber tubing to an empty chamber having an exhaust nozzle made of a 1% inch bore glass tubing 30 inches long. The smoke leaving the exhaust nozzle of the empty chamber was of a typical blue color and possessed a characteristic smoke odor. The chamber was then filled with glass fibers coated with tricresyl phosphate taken from a commercial air filter and the smokefilled air passed through the exhaust nozzle. No change in the color or odor of the smoke was observed in comparison to the color and odor observed in the test using the empty chamber.A The glass fiber filter was then replaced with shredded polyethylene of the type used in Example 1 and the smoke-filled air passed therethrough, and it was observed that the blue color of the smoke disappeared and the odor was much less pronounced, indicating that the solids present in the smoke were being removed by the polyethylene filter medium. To obtain a measurement of the amount of material being retained by the filter, the smoke emitted from the nozzle was passed through a cloth disc for 30 seconds and the residue weighed after drying to remove moisture. The results were as follows:

. Weight of Percent Enter Medlum Residue Retention 0.0202 0.0445 None Polyethylene O. 0012 94.

The following example shows in a different manner the relative efficiency of the filter of this invention in comparison with an impingement type of lter.

Example 4 filter was uniformly coated with black soot throughout the thickness of the filter.V This soot had passed through the glass fiber lter. Foreign matter retained by the glass fiber filter was examined and was made up primarily of larger particles of dust and lint.

The same test was repeated with the, exception that the filters were reversed and the air passed through the polyethylene filter first, and then through the glass fiber filter. After being in service for '786 hours, the filters were examined and the polyethylene filter was uniformly coated with black soot throughout its thickness and the glass ber filter from all appearances was substantially as free from foreign matter as it was when it was first put into use.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

I claim:

1. A gas filter for removing suspended particles of matter from a gas, which filter comprises polyethylene elements arranged to form a gas pervous mat of sunicient -thickness to permit multiple contact with the polyethylene elements of the particles of matter carried by the gas.

2. A gas filter according to claim 1 in which the polyethylene elements are distributed in random arrangement.

3. A gas filter according to claim 2 in which the polyethylene elements are in solid form.

4. A gas filter according to claim 3 in which he polyethylene elements are in the form of ribons.

5. A gas filter according to claim 4 in which the ribbons of polyethylene have a ragged edge capable of mechanically holding particles of matter coming in contact therewith.

6. A gas filter according to claim 3 in which the polyethylene elements are in the form of filaments.

7. A gas cleaning device for removing suspended particles of matter from a gas, which device comprises a gas pervous mat having gas contacting surfaces of polyethylene, said mat being of sumcient thickness to permit multiple contact with the polyethylene surfaces by the particles of matter carried by the gas.

8. A gas cleaning device for removing suspended particles of matter from a gas, which device comprises a gas permeable filter m-edium having gas contacting surfaces comprising polyethylene, said filter medium being of sufiicient thickness to permit multiple contact of the polyethylene surfaces with the particles of matter carried by the gas.

WILLIAM H. NICOL.

REFERENCES CITED The following references are of record in the le of this patent:

UNIIED STATES PATENTS Great Britain Sept. 26. 1938 

1. A GAS FILTER FOR REMOVING SUSPENDED PARTICLES OF MATTER FROM A GAS, WHICH FILTER COMPRISES POLYETHYLENE ELEMENTS ARRANGED TO FORM A GAS PERVIOUS MAT OF SUFFICIENT THICKNESS TO PERMIT MULTIPLE CONTACT WITH THE POLYETHYLENE ELEMENTS OF THE PARTICLES OF MATTER CARRIED BY THE GAS. 